An indoor unit of an air conditioner of the ceiling type is generally mounted on the ceiling of a room, with an air outlet and an air inlet disposed on the bottom of a casing of the indoor unit. Accordingly, air may return from the room below to the indoor unit via the air inlet, and be blown to the room via the air outlet. Due to the limited size of the indoor unit, the distance between the inlet 12 and the outlet is small and it is possible that partial blown air is drawn by the air inlet, thus causing a mixing of the blown air and the intake air. In particular, in a heating operation, heated air exiting via the air outlet is relatively lighter and tends to rise, and therefore is hard to be delivered to areas near the ground.
In order to deliver the heated air to areas near the ground, it is required to increase the air blowing speed and the static pressure so as to make the air blowing direction as vertical as possible. The heated air discharged via the air outlet in this condition, however, is much easier to be drawn by the air inlet, thus causing a mixing of the blowing air and the intake air much easier. Therefore, providing an indoor unit of an air conditioner of the ceiling type which can improve both the air blowing range and air blowing effect (especially improving air blowing effect in the heating operation) and reduce the mixing of the blown air and the intake air become a problem to be solved.